14.8.3 Milter Requirements and Usage Considerations

The Milter server can run on a separate system of its own, on the same system as Messaging Server, in a single system deployment, or, in a two-tier deployment, on the same system as the MTA. When LMTP is used between the MTA and the message store, the filtering must be invoked from the MTA, it can not be invoked from the message store. When SMTP is used between the MTA and the message store it can be invoked from either one, and it can run on either system or a third separate system.

Messaging Server supports connecting to multiple Milter servers. If you specify a domain name that translates to multiple IP addresses, the system will try all of them in order received from the DNS until one works. Some DNS servers support randomizing the order of addresses they return, so this provides a primitive load balancing/failover facility.

14.8.3.1 Supported Milter Message Modification Actions

The Milter interface currently supports the ability to add headers (SMFIF_ADDHDRS), change or delete headers (SMFIF_CHGHDRS), and quarantine messages (SMFIF_QUARANTINE). Changing the message body (SMFIF_CHGBODY), adding recipients (SMFIF_ADDRCPT) and deleting recipients (SMFIF_DELRCPT) are not supported at the present time.

14.8.3.2 Macros Provided by the Milter Interface

The following macros are currently defined by the Milter interface:

$j Text placed in the by clause of Received: header fields. In Messaging Server this is controlled by the RECEIVED_DOMAIN MTA option. If the option is not set, the official host on the local channel is used instead.

${client_addr} The IP address of the SMTP client, expressed as a dotted quad value. Only set when SMTP over TCP is being used.

$i Queue ID for the current message. Messaging Server generates a unique ID for each session; this ID is what appears in the $i macro.

${mail_addr} The MAIL FROM address for the current transaction.

${mail_host} The host part of the MAIL FROM address for the current transaction.

${rcpt_addr} A RCPT TO address for the current transaction.

${rcpt_host} The host part of the current RCPT TO address.